1. Field of the Invention
The present invention relates to an optical disc apparatus for recording and reproducing information to and from an optical disc; to a timing signal generator for generating a timing signal required for recording and reproducing information in an optical disc drive according to the present invention; and to an information recording and reproducing method for recording and reproducing information to and from an optical disc.
2. Description of the Related Art
Optical discs are widely used today as a high capacity information storage medium, and continue to be developed and marketed for use with computer storage peripherals and audio-visual recording.
An optical disc typically has a spiral track formed on the disc surface. Information is recorded and reproduced to and from the disc by means of a laser beam tracking and emitting a light spot to this track. The track is also typically divided into a plurality of sectors, which are the smallest units for recording and reproducing data on the disc.
FIG. 17 shows a typical example of a sector structure on a rewritable optical disc.
Each sector 1701 starts with a header area 1702 to which a signal containing address information is prerecorded by means of pits and lands in the optical disc surface. Though not shown in the figure, the header area 1702 may begin with a pattern of long marks, known as the sector mark, to make sector indexing easier using a pattern matching technique. The sector mark is used on, for example, rewritable optical discs conforming to the ISO/IEC 10090 standard. Alternatively, the header area 1702 may start with a repeating pattern of a specific period, known as a Variable Frequency Oscillator (VFO), for faster clock reproduction.
An optical disc drive typically manages the location of each data unit based on the address information in the header area 1702. It does this by indexing to a particular sector by detecting the sector mark pattern at the beginning of the header area 1702, or detecting a signal presence based on the envelope, and then reading the address information from the header area 1702. This also makes it possible to search and access data quickly and easily.
A gap 1703, data recording area 1704, and buffer area 1705 following the header area 1702 in succession. The gap 1703 is used, for example, for controlling the laser power for data recording and reproducing, and no data is recorded. The data recording area 1704 is where user data is actually recorded. Digital user data is recorded by adding an error correction code or other redundant checking code, and the modulating the digital data according to specific modulation rules for recording. The buffer area 1705 is provided as a means of absorbing variations in the rotational velocity of the disc, and has no signals recorded thereto.
Various methods are used for determining the sector arrangement, including constant linear velocity (CLV), constant angular velocity (CAV), an improved CAV method known as zoned CAV (ZCAV), and an improved CLV method known as zoned CLV (ZCLV).
In the CLV method, the disc motor is controlled so that the rotational velocity of the disc is inversely proportional to the disc radius, and the linear velocity of the recording and reproducing track (the "data track" below) is constant at any radial position on the disc. Disc storage capacity can thus be increased, and data can be recorded and reproduced synchronized to a constant clock. An advantage of the CLV method, therefore, is that substantially the same recording and reproducing conditions can be used at all track positions from the inside to the outside circumference of the disc.
In the CAV method, the disc motor speed and the recording/reproducing frequency remain constant. Rotational control is therefore simple, and a small motor can be used. The drawback to this method is that the total storage capacity of the disc is reduced.
The ZCAV method was developed to address the problem of low overall storage capacity while retaining the simple rotational control of the CAV method. In zoned CAV the disc speed remains constant, as in the CAV method, but the optical disc is divided radially into a plurality of zones, each containing a specific number of tracks. The recording/reproducing frequency is then changed in each zone from the inside circumference to the outside circumference of the disc. Storage capacity approaching that of CLV discs can thus be achieved.
The ZCLV method addresses the greater complexity of motor control required for CLV control. That is, the recording/reproducing frequency is kept constant, the disc is divided into a plurality of zones similarly to the CAV method, and the angular velocity of the disc is decreased in each zone from the inside circumference to the outside circumference of the disc. Storage capacity therefore remains comparable to that of the CLV method while disc motor control is simplified compared with CLV control.
Various methods of deriving a reference clock have been proposed for rewritable optical disc formats. These include a so-called sample servo method in which pits are preformed at a constant interval in a zigzag pattern centered on the track center, and these "clock pits" are detected to generate a reference clock for data recording and reproducing. Another method is to form a "wobble groove", an undulating sine wave shaped groove, centered on the track center, and reproduce a wobble signal recorded to this wobble groove to generate the reference clock for data recording and reproducing.
The period of the undulations in the wobble groove is normally sufficiently longer than the channel bit period of the recorded data, and is typically an integer multiple of the channel pit period. This improves the frequency divisibility of the data reproduction signal with the wobble signal, and makes it relatively simple to derive a reference clock.
A wobble groove can be used together with a sector structure, as shown in FIG. 17, having a header area in which address information is recorded by preformed pits. A land and groove recording method whereby address information is recorded using pits in the header area, a wobble groove is formed in the data recording area, and user data is recorded using both the groove and areas between adjacent grooves ("lands"), has been recently proposed. This land and groove method is currently used as a recording method for recent high density rewritable optical discs in combination with the above-described ZCLV control method.
To record and reproduce data using an optical disc with a sector structure as described above, common optical disc drives typically generate gate signals, such as a write gate and read gate, and use the timing derived from these gate signals to determine the beginning and end of data recording and reproducing. Such conventional drives further comprise a counter for counting the reference clock, and determine the gate signal timing based on the generated count.
With the ZCLV control method described above, however, there is a certain delay required for the disc motor to settle to the specific speed when searching between zones because the angular velocity of the disc changes from zone to zone. This means that when timing is based on a reference clock, data recording and reproducing cannot start until the disc motor stabilizes at a rotational speed corresponding to the reference clock.
Furthermore, it is also necessary to change the frequency of the clock used as the reference for recording and reproducing when the optical disc drive starts up, and when searching across zones, when using a ZCAV optical disc because the recording/reproducing frequency differs from zone to zone.
The time required for the optical disc drive to start up, and the time required for disc searches, is thus increased (slowed) when using ZCLV optical discs and when using ZCAV optical discs.